Limewash
and Distempers

Elizabeth Hirst

Ochre
limewash on the infill panels of the timber framed pub at Naunton
in the Cotswolds, which is owned by the National Trust

For many years the
use of traditional lime-based paints was restricted to those involved
in the conservation industry, but more recently these paints have enjoyed
a renaissance with the consumer due to increased publicity in interior
design and home magazines. Their textured, matt finish and distinctive
pastel colours provide an interesting alternative to modern paints, and
they are particularly in keeping with the character of old and vernacular
buildings.

These paints have
much more to commend them than their appearance, as their unique properties
make them suitable for various remedial solutions. In particular, their
high porosity and permeability enables walls to breathe, reducing the
risk of damp.

Externally, limewashes
were the most common choice for the walls of buildings particularly in
rural areas and small towns prior to the development of modern paint systems.
Lead paints were mostly used on joinery and metalwork but are also found
on exterior walls of the 18th and 19th centuries.

Internally, limewashes
and distempers were used to decorate the walls of most ordinary houses
and all but the principal spaces of the most important houses. Even fine
wall paintings and decorative schemes found in 17th century interiors
such as the Merchant's House in Marlborough (see illustration) were commonly
executed in lime-based paints.

LIMEWASH

The
basic material of limewash, lime, is derived from limestone or chalk (both
are forms of calcium carbonate) which is 'burnt' in a kiln to form quick
lime (calcium oxide). This highly reactive material is then 'slaked' by
adding water, a process which produces a great deal of heat, causing the
water to boil violently. The resulting material (calcium hydroxide) is
known rather confusingly as 'slaked lime', 'hydrated lime' and 'non-hydraulic:
lime', or even simply as 'lime'. In the wet form in which it is usually
kept it is also know as 'lime putty'. To make limewash this material is
then further diluted with water to form a thin paint of brushable consistency.

Slaked
lime is the principal active ingredient of both limewashes and mortars.
It hardens as it dries out by reacting with carbon dioxide in the air,
slowly reverting to the stone form of lime, calcium carbonate. Additional
binders are frequently added to limewash for a number of reasons, for
example to provide greater adhesion and binding of the paint layer, more
flexibility, and variations in porosity.

Different
quarry sources provide different qualities of lime; high calcium (pure
lime) is usually preferred for its consistency of material and its white
colour. Other limes may contain impurities and be inconsistent, but can
also be used to produce lime paints and may be white or off-white in colour.

As
slaking quick lime is a hazardous operation, current awareness of health
and safety issues means that this process is less frequently specified
by architects. Most now favour the use of the ready mixed product which
is available through specialist suppliers.

The
two types of limewash most frequently specified are tallow and casein.

Tallow limewash

Tallow
is a type of animal fat. It is incorporated during the slaking process
as the heat which is produced melts the fat enabling it to be evenly dispersed
throughout the product.

It can be used on either interior or exterior surfaces, but it is more popular for exterior use as it is slightly less
porous than other forms of limewash and can prevent excessive water penetration.
It also tends to brush off on clothes, making it less popular for interior
use than some other types of limewash, such as casein.

Other
oils and waxes can be incorporated as an alternative to tallow. Linseed
oil is a popular alternative as it provides durability, some flexibility
and better adhesion to some substrates.

Casein
limewash

Casein
is essentially the solid component of milk. Traditionally
it was precipitated from skimmed milk by adding a weak acid. The material
reacts with the lime to form a calcium caseinate, improving the binding
capacity of the limewash. It can be applied to the inside and outside
of buildings but is particularly popular for interior use as it adheres
better and is more porous than a pure calcium carbonate paint layer.

In
its wet form casein limewash is particularly susceptible to mould formation
so a biocide should be added if the mixed product is to be stored for
long.

Other variations

Hydraulic
limewash may be desirable where setting and hardening is required in wet
conditions, as it sets by chemical reaction with water. This product can
be made with or without additives by mixing hydraulic lime powder and
water. (This product should not be confused with hydrated lime, which
is a dry powdered form of calcium hydroxide.)

Modified
limewash may incorporate a variety of one or more additives such as oil,
resin, wax, polyvinyl acetate or cellulose.

Limewash
is compatible with a wide range of building surfaces, including brick,
plaster and stonework. It was used historically as a readily available,
easy to use and cheap product, making it popular for outbuildings as well
as for domestic use. Its slight antiseptic properties also made it desirable
as it prevented mould and discouraged insects and vermin.

Limewash's
tolerance of damp substrates or newly carbonating plaster was also recognised.
Its use is still favoured on damp walls because it has a vapour permeability
of at least ten times that of modern resin paint. This allows a faster
evacuation of moisture and can minimise the deterioration of the surface
and substrate which can be caused by trapping of moisture.

Limewash
is used in the conservation of historic building fabric for all the reasons
mentioned above and, depending on its formulation, can be used as a consolidating
and sacrificial material on crumbling limestone. To simulate the appearance
of stone the lime-based coating is frequently tinted with finely crushed
stone.

Application

The
surface should be brushed or washed down to remove any loose particles
and sprayed or brushed with water to reduce suction. Old limewash or lime
plaster will need more damping down that less porous surfaces such as
stonework.

The
paint is usually applied thinly by brush although airless sprays can be
used. It is sem-transparent when first applied and then dries to form
an opaque coating. Problems are frequently encountered when decorators
try to brush on a visible, thicker layer, as this is likely to craze and
crack on drying. It is important that the material is thoroughly brushed
out and worked to a wet edge, as limewash abutting a dry edge will become
more apparent on drying. Each coat is affected by the suction of the underlying
layer and each subsequent layer tends to become lighter on drying; an
effect which is particularly noticeable with pigmented limewashes.

Smooth
surfaces such as polished limeplaster or gypsum may require a bespoke
primer to provide a key for the paint, which can be susceptible to failure
through lack of adhesion in these conditions.

DISTEMPER

Glue
bound distemper, or 'soft' distemper as it is sometimes called, is one
of the most porous forms of distemper paint. It is usually produced from
powdered chalk (calcium carbonate) and size (gelatin) such as rabbit skin
glue. According to Ian Bristow (see recommended reading), alternatives
to chalk have included lime (calcium hydroxide), white lead (lead carbonate)
and satin white (made by adding alum or aluminium sulphate to slaked lime).
Synthetic glues such as polyvinyl acetate and polyvinyl alcohol can be
used instead of animal glue.

This
material was often specified to provide a temporary decorative scheme
on new lime plaster which is still carbonating, because of its vapour
permeability. However, it is inappropriate for walls which have continual
damp problems, as eventually the glue binder will become depleted.

Distemper
is generally used on sound plaster, wallpaper or previously painted inside
surfaces. It is easily marked and discoloured, and it cannot be washed
down, which makes its use favoured for ceilings or on wallpaper, but not
in children's rooms! It can provide subtle tints and enhances the walls
of many fine drawing rooms.

Oil bound distemper

The
addition of oil with an emulsifier such as borax (sodium borate) created
a more durable and often washable distemper depending on its formulation,
which can be considered a predecessor of modern resin based emulsions.
It became popular as an interior paint in the early 19th century when
a number of manufacturers began to produce it commercially, and it remained
in common use until after the Second World War when it was finally superseded
by vinyl and acrylic emulsion paints. It is now popular once again for
its visual qualities.

Although
less porous than limewash, oil bound distemper retains excellent porosity,
and it is suitable for a variety of interior surfaces including timber.
This material may be known by different names depending on its ingredients:
modified limewash, water paint or, if it contains casein, milk paint.
The additives are generally the same as those used in limewash. Traditional
recipes that utilise casein have included lime (alkali) to dissolve the
casein. (Water is lost in drying and the casein hardens to its insoluble
form.) Other variations include the use of alkali silicates to emulsify
oil, and plasticisers such as glycerine may be added. In recent years
other products such as anti?foaming agents have sometimes been incorporated.
Traditional recipes are interesting but the reasons for the incorporation
of some additives is not always clear; some recipes may have been regionally
produced and passed from generation to generation without being questioned.

It
should be remembered that limewashes and distempers, with the exception
of glue bound distemper, are generally insoluble when dry.

Application

Glue
bound distemper is mixed with water and left overnight to allow for gelling.
It should be used on a clean, dry surface and previously soft distempered
surfaces must be thoroughly removed by washing down with warm water. Uneven
or high suction surfaces may be given a coat of claircolle (or clear coat),
which is a size coating with some water or pigment added.

Distemper
should not be watered down prior to painting as it needs to be applied
in its gelled state. It is usually applied by six-inch brush in vertical
strips, cutting in the leading edge, the corners, top and bottom edges.
It should be flowed on in one direction, finishing top to bottom in one
stroke. The leading edge should be kept open and an entire flank or surface
finished without stopping or allowing a dry edge. Never brush out or cross
the dry work.

PIGMENTS

For
both limewashes and distempers, pigments or colouring material may be
incorporated from animal, vegetable or mineral sources, provided that
they are alkali resistant. Lamp black and natural earth pigments were
commonly used in the past as they were cheap and readily available. These
included a wide variety of ochres such as sienna, burnt sienna, umbers
and yellow ochre. Blue limewash rarely appears until the Victorian period,
following the introduction of the first alkali resistant blue pigment,
French ultramarine, in the 1830s. Nevertheless, pigments which were not
alkali-resistant such as Prussian blue and indigo were also sometimes
used in external limewashes which, in any case, have to be renewed regularly.

MODERN
TECHNOLOGY?

Weakly
bound modern matt emulsions are frequently labelled with 'period' names
to entice many to decorate and recreate a fashionable 'traditional' interior.
In effect they are frequently using matt alkyd resin based paint with
reduced vapour permeability and a restricted palette of colours. Unfortunately,
many a proud owner may be disappointed with this alternative material,
which fails to mimic the subtle texture and effect for which they were
aiming.

Where
the external walls of historic buildings can only be painted from scaffolding
it makes sense to consider modern alternatives that will last longer than
limewash. Silicate paint systems such as Keim provide an alternative as,
like limewash, they also allow the structure to breathe and are alkali
resistant, but last for decades.

This
article is reproduced fromThe Building Conservation Directory, 2002

Author

ELIZABETH
HIRST (AMUKIC) is the principal conservator and partner of Hirst Conservation
and her work involves a wide range of conser-vation disciplines. She
is a director of the subsidiary company Hirst Conservation Materials
Ltd, which researches and supplies a wide range of paints and coatings.